H2O2 produced by viridans group streptococci may contribute to inhibition of methicillin-resistant Staphylococcus aureus colonization of oral cavities in newborns

In an accompanying report, we showed that viridans group streptococci may p revent methicillin-resistant Staphylococcus aureus (MRSA) colonization of t he oral cavities of newborns. In the present study, we investigated the mec hanism of prevention in vitro. Most viridans group streptococci had bacteri ocin-like activity and killed MRSA, Burkholderia cepacia, Enterobacter aero genes, and Pseudomonas aeruginosa; however, Escherichia coli, Enterobacter cloacae, and Candida albicans were resistant. The activity was induced only by H2O2-secreting strains and was inhibited by horseradish peroxidase or c atalase in a dose-dependent manner. The mean concentration of H2O2 produced by 18 strains of viridans group streptococci (1 x 10(8) cfu in 200 muL of culture medium +/- standard deviation was 1.24 +/- 0.60 mmol. Viridans grou p streptococci inhibited MRSA growth in saliva as well as in culture media. These results indicate that H2O2 produced by viridans group streptococci m ay inhibit MRSA colonization of oral cavities in newborns.